A galaxy thought to be over the hill
is apparently still
hard at work creating baby stars, a new study finds.

Photos taken by the Hubble Space
Telescope show the core of
an elliptical galaxy known as NGC 4150, which was thought to
be beyond its
fertile years for star formation, awash in streamers of dust, gas and
clumps of
young, blue stars that are significantly less than 1 billion years old.
[Hubble
photo of the galaxy NGC 4150]

The observations suggest that elliptical
galaxies like this still have some youthful vigor left,
thanks to
encounters with smaller galaxies. The evidence also suggests that the
star
birth in NGC 4150was sparked by a collision and
merger with a dwarf
galaxy.

"Elliptical galaxies were thought to
have made all of
their stars billions of years ago," said astronomer Mark Crockett of
the
University of Oxford, leader of the Hubble observations, in a release.
"They had consumed all their gas to make new stars. Now we are finding
evidence of star birth in many elliptical galaxies, fueled mostly by
cannibalizing smaller galaxies."

The findings could change
astronomers' views of how these
galaxies form, he said.

"These observations support the
theory that galaxies
built themselves up over billions of years by collisions with dwarf
galaxies," Crockett said. "NGC 4150 is a dramatic example in our
galactic backyard of a common occurrence in the early universe."

Astronomers used Hubble telescope's
powerful Wide
Field
Camera 3 to observe NGC 4150The Hubble
images reveal turbulent activity deep inside the elliptical
galaxy's core.
Clusters of young, blue stars trace a ring around a center that is
rotating
with the galaxy. The stellar breeding ground is about 1,300 light-years
across.
Long strands of dust are silhouetted against the yellowish core, which
is
composed of populations of older stars.

From a Hubble analysis of the stars'
colors, Crockett and
his team calculated that the star-formation
boom started about a billion years ago, a comparatively
recent event in
cosmological history. The galaxy's star-making factory has slowed since
then.

"We are seeing this galaxy after the
major starburst
has occurred," explained team member Joseph Silk of the University of
Oxford. "The most massive stars are already gone. The youngest stars
are
between 50 million and 300 to 400 million years old. By comparison,
most of the
stars in the galaxy are around 10 billion years old."

The team's results will be published
in an upcoming issue of
the Astrophysical Journal.